U.S. Pat. No. 4,131,446 issued on Dec. 26, 1978 for a "Method and Apparatus for Manufacturing Flat Glass on Molten Metal."
Basically, this patent discloses a flat glass manufacturing process in which the glass is developed by a float process which occurs on a molten metal bath. In such a glass manufacturing process, there is a forward flow of molten metal along the length of the bath. This flow is caused because the upper part of the molten metal is entrained by accelerating glass thereover. This forward flow moves over a return flow of cooler molten metal which is moving in a deeper region of the bath.
The patent teaches a construction in which cooler return molten metal flow is drawn from a deeper, colder region of the bath to replenish the forward flow of molten metal entrained by the accelerating ribbon which is moved from an entry end of the bath to an exit end thereof. The patent teaches the use of a barrier 35 which extends partially across the float glass chamber at that portion of the chamber where the stretching of the glass is carried out to reduce the glass to its final width dimension. The barrier 35 is best shown in FIG. 3 of the patent and best described in the patent commencing at line 6 of column 6.
The barrier 35 extends substantially most of the distance across the width of the float glass chamber, but does not extend into contact with the side walls of the chamber. This permits flow between the ends of the barrier and the side walls of the chamber. The flow permitted in this region is a return flow of cooler tin from the exit end of the chamber towards the hot end of the chamber, that is the entry end of the chamber through which molten glass is poured on the molten metal.
As is shown in the drawings of this patent, the barrier 36 has a relatively flat face facing against the direction of movement of the ribbon of glass which is moving thereover. Column 6 of this patent, at line 13, indicates, "The barrier 35 insures that the lower layers of entrained molten metal of the forward flow are directed downwardly and then upstream as indicated by arrow 38 in FIG. 3." Arrow 38 of FIG. 3 indicates that the molten tin is essentially sheared off from the moving ribbon of glass 8 and doubled back against the direction of movement of the glass ribbon thereover.
At column 6, line 43 of the patent, it is further stated, "From the foregoing, it is seen that the barrier 35 is at a location in the region of the downstream end of the attenuation zone and constrains molten metal flow at that location to forward flow entrained beneath the ribbon and counterflow alongside the ribbon from downstream of that location. The barrier 35 thus obstructs direct return flow of molten metal along the bath bottom into the region upstream of the barrier location, but permits counterflow around the ends of the barrier from the region of greater bath depth thereby establishing lateral access to the region of the bath supporting ribbon as it is being attenuated by acceleration of the glass upstream of the barrier location."
We have found that the flat front face construction of the barrier 35 shown in the patent allows cold return tin to immediately move under the ribbon of glass in a turbulent manner. This turbulent flow penetrates into the forward flowing molten tin thereby causing local, variable chilling of the bottom surface of the glass ribbon. This action also does not provide for a good intermixing of the cold tin flowing upstream with the warmer tin which has been sheared off of the glass ribbon passing over the top of the barrier.
The principal objects of this invention are to provide a barrier structure or dam for a float glass chamber which provides an outwardly flowing force of hot tin from the dam which prevents cold tin from flowing under the moving glass ribbon and which insures a thorough intermixing of molten tin sheared off of the tin currents moving wth a moving glass ribbon and tin returning from a downstream location in the float chamber.
It is still an additional object of this invention to provide such a barrier structure or dam for a float glass chamber which insures the mixing of warm and cold tin currents at a location spaced outwardly from underneath the ribbon of glass moving over the dam so that the ribbon of glass is not subjected to tin of varying temperatures which could cause optical distortion in the glass ribbon being manufactured.
U.S. Pat. No. 4,131,446 appears to contain the most relevant description of dams used in float glass chambers for controlling tin currents. Many other patents show or briefly discuss dams or other devices for controlling tin currents. Other patents which may be of interest include: U.S. Pat. Nos. 3,615,315; 3,770,407; 3,790,361; 3,930,828; 3,930,829 and 3,954,432. These patents are not as relevant to the subject matter of this specification as the aforementioned U.S. Pat. No. 4,131,446.